Fix mingw32 thiscall + sret.

Unlike msvc, when handling a thiscall + sret gcc will
* Put the sret in %ecx
* Put the this pointer is (%esp)

This fixes, for example, calling stringstream::str.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196312 91177308-0d34-0410-b5e6-96231b3b80d8